I made some further measurements on another LM2596 converter module and it seems to me that there is a built-in diode from output (anode) to input (cathode) inside the LM2596 chip. This would explain the large current from connected battery at the output to a zero impedance input connection which blew up the LM2596 chip and the catch diode. A diode from output to input is not shown on the block diagram in the LM2596 datasheet. Could you please comment on my observation ?
Thank you in advance.